Hi Czar,
Regarding the cause of the symptoms: The spinal degenerative process explains the loss of feeling, weakness and muscle wastage in the hind legs. This in turn will explain the swaying and knuckling.
Regarding the cause of the disease itself: There is no known cause for the disease although the very strong breed predisposition would suggests a genetic origin.
Regarding "why rule out the others if not tested": I do not understand your question. Sorry.
I hope this helps. Best wishes, Julien
CDRM: Symptoms, Diagnosis and Treatment